It is known that a disk-shaped ferromagnetic material having a diameter of approximately a few μm and thickness of a few dozen nm has a magnetic vortex structure in which a magnetic structure winds around the circumference of the disk. In the present invention, a ferromagnetic material having such a structure is called “a ferromagnetic dot.” In the vicinity of the center of this ferromagnetic dot, a magnetization rises, in a microscopic area with a diameter of approximately 10 nm, in a direction perpendicular to the plane of the disk (for example, refer to Non-Patent Document 1). In the present invention, the area where the magnetization rises in a perpendicular direction is called “a core.”
Previous researches have revealed that if a magnetic field is applied to such a ferromagnetic dot in a radial direction of the dot, the position of the ferromagnetic dot's core changes and if the application of the magnetic field is halted, the core while exhibiting a spiral motion returns to the center of the ferromagnetic dot. In addition, it is known that if the magnetic field to be applied is an alternating current magnetic field which resonates with the rotational period of the core, the core exhibits a rotational motion with a given diameter in the plane of the ferromagnetic dot (for example, refer to Non-Patent Document 2).    [Non-Patent Document 1] Shinjo, T., et al., “Magnetic vortex core observation in circular dots of permalloy,” Science, 289, 930-932 (2000).    [Non-Patent Document 2] Guslienko, K. Yu, et al., “Eigenfrequencies of vortex state excitations in magnetic submicron-size disks,” J. Appl. Phys., 91, 8037-8039 (2002).    [Non-Patent Document 3] R. P. Cowburn, et al., “Single-Domain Circular Nanomagnets,” Physical review letters 83, No 5, 1042-1045 (1999).    [Non-Patent Document 4] 25. Shibata, J., et al., “Current-induced magnetic vortex motion by spin-transfer torque,” Phys. Rev., B 73, 020403 (2006).